tata steel' s experiences

National Workshop on Boiler Corrosion , 11-12th April, 1995, NML Jamshedpur, INDIA

WATER SIDE CORROSION IN BOILERS -TATA STEEL' S EXPERIENCES

N. KRISHNASWAMY AND P.S. BHATTACHARYATata Steel , Jamshedpur - 831 001

Introduction

Captive power plants in an inte-

grated steel Plant like Tata Steel

have got two basic functions to per-form, viz. meeting the essential

power requirement and supplying

the process steam. Process steam

requirement gets the first priority

and has to be met under all condi-

tions. Even though the steel plant

may have a tie-up with the utility for

getting a certain contracted power

supply, the captive power plants

have to produce the base load-thesum of the most essential loads which

cannot be shed-off from equipment

and process safety point of view-

after meeting process steam demand.

Under the present scenario of eco-

nomic liberalisation, the cost of op-

erations has to be brought down and

kept at the lowest levels dictated by

market forces. The capitve power

plants can meet all these objectives

only if all the boilers are operated at

a high efficiency, availability and

reliability.

Corrosion and other related prob-lems are of great concern to powerplant operators. A proper under-standing of these aspects will go along way in achieving the perfor-mance levels expected from theboilers and the machines they serve.

This paper attempts to throw somelight on some of the important areaswhich a boiler engineer must knowfor getting a high level of perfor-mance from the boilers.

Iron Contamination In Boilers -The Sources, The Problem And

Its Control

It is well known to all boiler op-

erators that many problems in boil-

ers are directly related to corrosion

or deposition of both soluble andinsoluble iron and considerable time,

effort and money are spent in fight-

ing the battle of iron. Hence a sound

understanding of the nature and

sources of iron and the methodology

of preventing its deposition on heat

transfer surfaces is essential for ef-

ficient and reliable boiler operation.

Sources of Iron

Iron compounds contaminate theboiler from installation . The millscale on the plates and pipes, weldslag and splatter, etc., are the initialsource of iron, and if not removedcompletely, it will start a cycle ofiron fouling. Initial hydrostatic testsare often carried out using oxygenladen water which results in the for-mation of rust on the

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N. KRISHNASWAMY AND P.S. BHATTACHARYA

J

boiler internal surfaces. Improper

lay-up procedures followed during

long lay-off of a boiler also bring in

significant quantity of iron into the

boiler. During normal operation,

corrosion in pre-boiler, boiler and

after-boiler systems produce oxides

of iron in varying degrees depend-

ing upon the competency of opera-

tion. Although boiler manufactur-

ers spell out procedures for boiler

start-up and shutdown, these are not

always followed exactly as given.

Sudden thermal expansions and con-

tractions fracture the protective mag-

netite film leading to further corro-

sion and generating more oxide.

Although excessive iron can

cause many problems, the worst is

the formation of a constantly grow-

ing insulating deposit- on a boiler

tube in high heat flux area. If al-

lowed to continue, failure of the tube

will result due to overheating.

It is often said that a boiler is

merely a film of magnetite supported

by structural steel. The ideal mag-

netite film is thin, adherent and of

constant thickness and texture. It is

this film that protects the boiler dur-

ing operation from further corro-

sion. Deposits of iron can destroy

this protective film.

Salts present in boiler water can

concentrate beneath the iron deposit

and this too can cause failure through

many mechanisms such as caustic

corrosion, low pH corrosion, hydro-

gen damage, etc.

Transported iron oxide can

foul Ion Exchange resin beds but

this contamination can be controlledwith proper cleaning of the resin.

Controlling the Problem of Iron

,With boiler systems demand-ing greater reliability and efficiency,iron contamination heads the list ofoperator's concerns. Good practicesand proper treatment can bring itunder control. Some of the prac-tices, followed with benefit in TataSteel to combat this problem, are :

I. Precommissioning chemi-

cal cleaning of pre-boiler,

boiler and some of the af-

ter-boiler systems.

For the intermediate pres-

sure boilers that we have,

this was not considered es-

sential some fifteen years.

back. But now , this is con-sidered as an essential stepin commissioning.

2. Proper commissioning toensure clean passivated in-ternal surfaces in the boiler.

3. Maintenance of make-upwater quality stringently.

As our boilers have to supply a

significant quantity of process steam

to the steel plant, DM water make-

up is of the order 40 to 45%. Com-

promise in quality of make-up can

lead to very serious problems reduc-

ing availability of boilers and tur-bines which we can ill-afford.

4. Use of stainless steel lines totransfer DM water to deaerators.

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5. Select deaerators of provendesign and monitor perfor-mance constantly.

The performace of a deaerator

is as good as its design and hence

a lot of care has to be taken while

selecting the design and make.

Spray type deaerators give more

reliable and better performacne

than try type.

6. Chemical cleaning ofoperating boilers basedon deposit density moni-toring.

Boiler Corrosion

To maintain reliable boiler op-eration, corrosion of boiler metalby the water used to generate steammust be prevented. Dissolved oxy-gen, high temperatures, and acidand caustic in the water can, ifuncontrolled, severely pit, gougeor embrittle boiler tube metal andeventually lead to tube failure. Itis necessary to understand the typesof corrosion that can occur, theircauses and - more importantly -their prevention.

It is a good practice to takesample pieces of furnace tubes fromhigh heat flux areas during annualturnarounds to monitor deposit

density and chemical compositionand plan chemical cleaning when

density exceeds 40 mg/cm2. With

good operating practices , we find

need to acid clean the boilers oncein about twelve years.

7. A proper feed water con-ditioning program to con-trol corrosion in pre-boiler, boiler and after-boiler systems and peri-odic audit of the same byan expert agency.

8. Proper storage of idle boil-ers using either wet or drymethod.

In Tata Steel, we do not facethis situation normally, as boilers,whenever they are available, arekept in operation to meet powerand process steam requirements.

Corrosion Essentialto boiler operation

Since no oxygen is normallypresent, the primary corrosion re-action in an operating boiler is :

3 Fe + 4H20 -p Fe304 + 4H2Iron Water Magnetitie Hydro-

or steam gen Gas

The product of this process isblack iron oxide or magnetitie thatbuilds a tenacious layer at the steelsurface. When the magnetite layergrows to a thickness of 0.0002 in.to 0.0007 in., damaging corrosionstops. Under changing tempera-tures and dynamic boiling envi-ronment, some damage takes placeto this protective layer which isagain restored. The normal corro-sion progresses slowly at approxi-mately 1 mil. (0.001 in) per year.The primary objective of boilerwater treatment is to promote con-ditions that preserve the magnetitelayer.

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Types of Corrosion

1. Caustic Corrosion

The stability of magnetite layer

is highly dependent on the pH of the

boiler water. Fig. I shows the effect

of pH on boiler corrosion or more

accurately, magnetite dissolution. It

is dissolved by both low pH and

high pH environments.

The terms 'Caustic Gouging'

and 'Ductile Gouging' refer to the

corrosion under the action of

localised concentrated sodium hy-

droxide. It is commonly encoun-

tered in :

* Water-cooled tubes in regionsof high heat flux.

* Slanted or horizontal tubes.

* Locations beneath heavydeposits.

* Heat transfer regions at or adja-

cent to backing rings at welds or

to otherdevices that disrupt flow.

Two critical factors that con-

tribute to caustic corrosion are avail-ability of sodium hydroxide or of

alkaline-producing salts and themechanism for concentration.

Some of the Mechanisms of concen-

tration are

* Departure from nucleate

boiling (DNB) : Under DNB

condition, a stable film or

blanket of steam forms on

the tube surface. Corrosives

concentrate at the edges ofthis blanket.

* Under-deposit concentra-tion. Fig. 2 explains the

mechanism of under - depositconcentration.

* Evaporation at a water line: When a waterline exists,corrosives may concentrateby evaporation resulting ingouging along the waterline.

* Indications of caustic cor-

rosion : Hemispherical or

elliptical depressions, filled

with corrosion products and

sometimes containing spar-

kling crystals of magnetite.

At times, there may be a crust

of hard deposits and corro-

sion products containing

magnetite crystals. The af-

fected metal surface gener-

ally has a smooth, rolling

contour.

Elimination

* Reduce the amount of avail-able free NaOH.

* Prevent inadvertent release

of caustic regeneration

chemicals from DM Plant.

* Prevent in-leakages of alka-

line-producing salts intocondensers.

* Prevent DNB

* Prevent excessive watersidedeposition

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* Prevent the creation ofwaterlines in tubes

* Prevent process contamina-

tion.

2. Low pH Corrosion

A general depression of bulk-water pH may, although very rare,occur if certain acidic contaminantsenter the boiler. The more commonlow-pH attack arises due to localisedconditions. Two circumstances mustexist simultaneously to produce thiscondition :

* Availability of acid produc-

ing salts or free acid. Acid

producing salts may find

way through condenser

leakage. Unitentional addi-

tions of free acid may arise

from inadvertent release of

acid regeneration chemicals

into DM water make-up.

followed during acid cleaning, cor-rosion of internal surfaces of a

boiler will result from low-pH ex-

posure. Thermally induced break-

down of inhibitor, inappropriate

selection of cleaning agent or its

strength, excessive exposure times,

excessive exposure temperatures,

failure to completely neutralise,

etc. are some of the factors that

lead to uncontrolled acid corro-sion.

One of the first areas to be af-

fected is the tube ends inside the

mud and steam drums. Hand-hole

covers, drum manholes, and shell

welds may also be affected. Heat

transfer surfaces and weldments may

experience vigorous attack.

Shielded regions within crevices and

under remaining deposits may pre-

vent proper neutralisation of clean-

ing acid. This results in vigorous

localised attacks when the boiler is

returned to service.

* Mechanism of concentra-

tion : This is same as in caus-tic corrosion.

To eliminate low pH attack, theabove two conditions must be tack-led. It is very difficult to distinguishlow-pH attack and caustic corro-sion as in the locations where theyare found and in visual appearancethey are the same. However, metal-lographic examination will distin-guish the two.

3. Low-pH Corrosion During AcidCleaning

If propere procedures are not

4. Hydrogen Damage

Hydrogen demage may occurwhere corrosion reactions result inthe production of atomic hydrogen.Damage may result from high-pH orlow pH corrosion reactions. Atomichydrogen liberated diffuses into thesteel. Some of this diffused atomichydrogen will combine at the grainboundaries or inclusions in the metalto produce molecular hydrogen orwill react with iron carbides in themetal to produce methane. Thesegases accumulate at the grain bound-aries producing discontinuousintegranular micro cracks. Whensuch micro cracks accumulate, a part

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of the wall of the tube bursts giving

rise to a thick-walled failure.

Generally, hydrogen damage isconfined to water-cooled tubes inboilers operating above 1000 psi.Steps to eliminate caustic corrosionand low-pH attack will eliminatethe possibility of hydrogen damage.

5. Oxygen Corrosion

Oxygen corrosion is a complex

electrochemical reaction that results

in pitting in a small area of the metal

surface. If uncontrolled, it finally

leads to pin-hole type of failures of

tubes. Oxygen corrosion sites

present knoblike mounds of corro-

sion products that frequently cover

pits. These mounds are known as

tubercles which indicate presence

of oxygen corrosion.

The three critical factors for

the onset and progress of oxygen

corrosion are presence of moisture

of water, presence of dissolved oxy-

gen and an unprotected metal sur-

face.

Corrosiveness of water in-

creases as temperatue and dissolved

solids increase and as pH decreases.

Aggressiveness increases with an

increase in oxygen.

An unprotected metal surface

can be present under the following'

conditions :

* The metal surface is bare,

e.g., the surface after acid

cleaning without passiva-

tion.

* The metal surface is cov-ered with a marginally pro-tective or non-protectiveiron oxide such as hematite.

* Cracks exist in the protec-tive Magnetite film. Crack

occur due to mechanical and

thermal stresses during nor-

mal boiler operation, start-

up, shut down and rapid load

swings. When proper oper-

ating conditions are main-

tained, the cracks get re-

paired and magnetite film

reforms. However, if ex-

cessive levels of oxygen are

present , the cracks cannotbe adequately repaired and

corrosion commences.

Locations of Oxygen Corrosion : In anoperating boiler , the first areas to beaffected are the economiser and feedwater heaters . In cases of severe oxygencontamination , other areas such as wa-terline in the steam drum and steamseparation equipment are corroded.

Oxygen attack is more of a

problem in idle boilers. The entire

boiler system is susceptible but the

most common attack sites are thebends in superheater tubes wheremoisture can collect.

Control in Operating Boiler: Thisis effected by bringing down dis-solved oxygen to very low levels.

To achieve this dissolved oxy-gen level , an excess of chemicaldeaerant is needed in the boiler wa-ter. Recommended residuals are 20to 60 ppm of sulphite or 0.1 to 0.5

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ppm of hydrazine, depending uponthe scavenger used. Daily tests willshow the level of excess. Recordsshould be kept of the amount ofchemical deaerant fed. This recordprovides a constant efficiency checkof mechanical deaeration as well asindication of any infiltration of dis-solved oxygen into the system.

Possible causes of excessivelevels of dissolved oxygen are im-proper functioning of deaerator,improper feed of oxygen scaveng-ing chemicals and air in-leakage.Monitoring of oxygen levels at theeconomiser inlet, especially duringstart up and low load operation, isrecommended. Elimination of oxy-gen corrosion in an operating boileris achieved by looking at these as-pects very carefully and taking re-medial actions. The results thatcan be achieved could be surprisingin some cases. Table No. I indicateshow economiser leakages due tooxygen corrosion could be broughtunder control in two boilers of sameoperating life with the above men-tioned approach and in one case,costly replacement of completeeconomiser could be avoided withno difference in reliability.

Control in Idle Boiler: An idle boilerwill have to be protected by usingproper procedure of wet lay-up ordry lay-up if oxygen corrosion is tobe avoided. The wet lay-up proce-dure involves complete filling of theboiler with water of very low oxy-gen content, use of sufficiently highlevels of oxygen scavenging chemi-cals, maintenance of properly ad-justed pH levels and periodic water

circulation. Successful protectionof an idle boiler during dry lay-updepends upon consistent eliminationof moisture and/or oxygen involv-ing use of dessicants and nitrogenblanket or continuous circulation ofdry, dehumidified air (<30% RH).

Protection of a Boiler After AcidCleaning: This is achieved by de-veloping a protective magnetite filmcoating on the metal surfae throughrinsing followed by a post boilout-passivating step.

6. Corrosion due to Copper

Pitting of boiler drums and tubebanks has been encountered due tometallic copper deposits formedduring acid cleaning procedureswhich do not completely compen-sate for the amount of copper oxidesin the original deposits. Dissolvedcopper may be plated out on freshlycleaned steel surfaces, eventally es-tablishing anodic corrosion areas andforming pits very similar to oxygenin form and appearance. In mostcases, it is localised in certain tubebanks, giving rise to random pittingin those particular areas. Wheneverdeposits are formed containing largequantities of copper oxide or metal-lic copper, special precautions arerequired to prevent plating out ofcopper during cleaning operations.

7. Corrosion Fatigue Cracking

Corrosion fatigue failures mostfrequently occur in boilers that arein "peaking" service. used discon-tinuously or otherwise operated cy-clically. Rapid start -up or shutdown

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can greatly increase the possibility

of corrosion fatigue.

Corrosion fatigue refers to

cracks propagating through a metal

as a result of cyclic tensile stresses

operating in an environment that is

corrosive to the metal. Two com-

mon sources of cyclic tensile stresses

are cyclically fluctuating internal

pressure and constrained thermal

expansion and contraction. Opera-

tion at low-pH levels or with exces-

sively high levels of dissolved oxy-

gen may induce pitting. The pits act

as stress concentrators for the initia-

tion of corrosion-fatigue cracks.

The sequence of crack de-velopment is

* During first phase of cyclic

stress, the tithe wall under-

goes expansion when the

oxide layer which is brittle

may fracture.

* The exposed metal surfaceat the root of the crackoxidesess forming a micro-scopic notch in the metal sur-face.

* During the expansion cycle,

the oxide layer will tend to

fracture along this notch,

causing it to deepen.

* As this cyclic process con-tinues, a wedge shapedcrack propagates through thetithe wall until rupture occurs.

The cracks always propagate in a

direction perpendicular to the direction

of the principal stress. They are typi-

cally straight and unbranched, are

needle or wedge-shaped and propa-

gate perfpendicular to the metal sur-

face. They often occur in families of

parallel cracks.

Reduction or elimination of cor-rosion-fatigue cracking is achi-eved by controlling cyclic tensilestresses, controlling environmentalfactors and boiler design.

Conclusion

Of all the problems in boiler

operation, corrosion and corrosion-

related problems are, by far, the most

serious. If they are allowed to con-

tinue unchecked, the direct and in-

direct monetary loss to theorganisation on account of them can

be very substantial. However, if

proper care is taken during

commisioning and subsequent op-

eration of the boiler, corrosion and

its effects can be kept at a low level

and a long, efficient and trouble-

free life of the boiler and all associ-

ated equipment can be ensured.

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Table - 1

Economiser leakage due to oxygen corrosionin Boilers No. 1 & 2 at Power House No. 4

Year of Commisioning : Boiler No. 1 1973

Boiler No. 2 1974

No. of oxygen corrosion leaks

Year Boiler No. I Boiler No. 2

1974 11975 -1976 -1977 -1978 -1979 21980 11981 4

2

2

Remarks

1Economiser sectionscoompletely replaced inBoiler No. 1 only in 1981.Modifications to deaeratorsand other operational con-trols to reduce dissolvedoxygen introuduced in bothboilers in 1981 and 1982.Decided not to replaceeconomiser sections in

1982 - - Boiler No. 21983 - -1984 1 -

1985 1 -1986 - -1987 - -1988 - 11989 - 11990 - -1991 - -

1992 - -1993 1 2

1994 - -

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rMge

2 3 4 5 6 7 S 9 10 ii 12 13 14

PH

Fig. 1 High or low pH boiler waterconditions corrode boiler steel

1 Liter 5ppm NaOH5 grams NaOH x 1000 ml Soln.106 gram Solution= 0.005 grams NaOHConcentrating Mechanism at theTube Wall0.1 ml solution containing

0.005 grams NaOH

0.005 grams NaOH0.1 ml Solution= 50,000 ppm

Fig. 2 Concentration of causticcan occur beneath deposition

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tata steel' s experiences

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